Oil-free operation of air compressors has the advantage that the compressed air, e.g. in air handtools used in the immediate vicinity of individuals, is free of oil. Such compressors in which the piston is not in contact with the wall of the cylinder are known. However oil-lubricated piston rods maintain the working clearance between these pistons and cylinders. The disadvantage of such compressors consists in that a considerable portion of the compressed gas leaks through the clearance between piston and cylinder. Oil-free compressors are also of importance in refrigerating systems. Due to the mixing enthalpy of two phases of refrigerant mixtures enormous thermodynamic losses occur, therefore oil-free centrifugal compressors are preferred in large systems. All small units, comprising reciprocating compressors require a mixture of a refrigerant (e.g. freon) and a lubricant, therefore efficiency being considerably lower. A further disadvantage of these units consists in that an oil film precipitates on the walls of the heat transmitting tubes, causing increase of temperature drop and consequently reducing Carnot efficiency.
Oil-free operation is of special importance also to combustion engines since engines having walls without cooling (known as adiabatic engines) have a higher thermodynamic effiency.
Although piston engines with gas-lubricated pistons are known they are not used to date, since the fluid dynamic forces of the reciprocating piston in its dead center positions become zero.
It has also been proposed to use pistons and cylinders made of ceramic materials, but also this development has not been put into practice yet.